Increased transmission capacity for a fiber-optic link

ABSTRACT

An optical communication system is provided. The optical communication system includes an optical fiber, an optical transmitter, and an optical receiver. The optical transmitter is coupled to the optical fiber. The optical transmitter is adapted to encode a pulse amplitude modulated optical signal based on at least two, independent input signals. The optical receiver is coupled to the optical fiber. The optical receiver is adapted to decode the pulse amplitude modulated optical signal to reproduce the at least two, independent input signals as output signals.

TECHNICAL FIELD

[0001] The present invention relates generally to the field oftelecommunications and, in particular, to increased transmissioncapacity for a fiber-optic link.

BACKGROUND

[0002] Telecommunications systems transmit data, e.g., voice video andother data, between equipment at various locations. This equipmentincludes user equipment, access equipment, switches, and otherconventional telecommunications equipment. Telecommunications systemstypically include a variety of transmission medium to transmit data toand from the equipment. For example, some systems transmit data over oneor more of coaxial cables, fiber optic cables, or other appropriatemedium.

[0003] Over time, service providers increase the capacity of theirsystems to keep up with an ever-increasing demand for access to thesystem. One typical technique for increasing the capacity of the systemis to increase the speed at which data is transmitted over the system.Unfortunately, when fiber-optic cables are used to transmit data, otheraspects of the transmission medium limit the effectiveness of theincreased speed. For example, the “dispersion” effect limits the abilityof the service provider to increase the speed of data carried over thefiber-optic cable. The dispersion effect occurs when the lighttransmitted over the cable broadens out to the point where theinformation carried by the light is corrupted. To compensate for thiseffect, conventionally, expensive dispersion compensation circuitry isincluded in the system. However, in some applications, the expense ofthis additional circuitry outweighs the benefits of the increased speedof transmission.

[0004] For the reasons stated above, and for other reasons stated belowwhich will become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art forincreased capacity in transmission systems without the use of expensivedispersion compensation circuitry.

SUMMARY

[0005] The above mentioned problems with telecommunications and otherproblems are addressed by embodiments of the present invention and willbe understood by reading and studying the following specification.Embodiments of the present invention encode multiple digital datastreams into a composite signal using, for example, pulse amplitudemodulation and transmitting the composite signal over a fiber-optic linkin order to improve the capacity of the link.

[0006] More particularly, in one embodiment, an optical communicationsystem is provided. The optical communication system includes an opticalfiber, an optical transmitter, and an optical receiver. The opticaltransmitter is coupled to the optical fiber. The optical transmitter isadapted to encode an optical signal based on at least two, independentinput signals. The optical receiver is coupled to the optical fiber. Theoptical receiver is adapted to decode the optical signal to reproducethe at least two, independent input signals as output signals.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a block diagram of an embodiment of a telecommunicationssystem that encodes multiple channels for transmission over a fiberoptic link according to the teachings of the present invention.

[0008]FIG. 2 is a schematic diagram of an embodiment of an encodercircuit for an optical transmitter according to the teachings of thepresent invention.

[0009] FIGS. 3A-3G are timing diagrams illustrating an example of theoperation of the encoder circuit of FIG. 2.

[0010]FIG. 4 is a schematic diagram of an embodiment of a decodercircuit for an optical receiver according to the teachings of thepresent invention.

[0011]FIG. 5 is a graph that illustrates one embodiment for signallevels used by an encoder/decoder in a telecommunications systemaccording to the teachings of present invention.

[0012] FIGS. 6A-6G are timing diagrams illustrating an example of theoperation of the decoder circuit of FIG. 4.

DETAILED DESCRIPTION

[0013] In the following detailed description, reference is made to theaccompanying drawings that form a part hereof, and in which is shown byway of illustration specific illustrative embodiments in which theinvention may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to practice theinvention, and it is to be understood that other embodiments may beutilized and that logical, mechanical and electrical changes may be madewithout departing from the spirit and scope of the present invention.The following detailed description is, therefore, not to be taken in alimiting sense.

[0014]FIG. 1 is a block diagram of an embodiment of a telecommunicationssystem, indicated generally at 100, that encodes multiple channels fortransmission over a fiber optic link according to the teachings of thepresent invention. System 100 includes optical transmitter 102 that iscoupled to optical receiver 104 over fiber-optic link 106.Advantageously, system 100 provides increased capacity over existingsystems by allowing multiple analog channels to be encoded andtransmitted over a single fiber-optic link 106 without substantialimpacts due to increasing dispersion effects and without substantiallyincreasing the speed of transmission. In one embodiment, system 100 usespulse amplitude modulation to encode the signals from multiple sourcesas a composite optical signal to be carried over fiber-optic link 106.For purposes of the specification, “pulse amplitude modulation” is atechnique in which the amplitude of pulses in the composite opticalsignal are determined based on values of bits from multiple sources.

[0015] Optical transmitter 102 is coupled to receive inputs fromplurality of independent, analog data sources at ports 108-1, . . . ,108-N. For example, in one embodiment, optical transmitter 102 iscoupled to receive analog RF signals. Optical transmitter 102 includesfront-end circuitry for processing the received analog signals. Thisfront-end circuitry is reproduced for each of the inputs at ports 108-1,. . . , 108-N. For simplicity in description, only the front-endcircuitry for port 108-1 is described in detail. It is understood thatthe front-end circuitry for the remaining ports is constructed andoperates in a similar manner and thus is not described here.

[0016] The front-end circuitry for port 108-1 includes analog inputcircuit 110, analog to digital (A/D) converter 112, field programmablegate array 114, and serializer 116. Analog input circuit 110 amplifiesthe input signal and performs Nyquist filtering before providing thesignal to A/D converter 112. A/D converter 112 converts the analog inputsignals from analog input circuit 110 into a corresponding digitalsignal for application to field programmable gate array 114. Fieldprogrammable gate array 114 performs operations such as framing, coding,and scrambling for the digital data. Once processed, the data isprovided to serializer 116 to convert the parallel data from fieldprogrammable gate array 114 into a serial bit stream.

[0017] The front-end circuitry of optical transmitter 102 performsconventional functions to prepare analog signals for digitaltransmission over an optical link. Thus, portions of the front-endcircuitry can be removed or modified as necessary when digital or othertypes of analog signals are to be transmitted. For example, A/Dconverters are not necessary if the signals receive at ports 108-1, . .. , 108-N are in the digital domain.

[0018] Encoder circuit 118 is coupled to the front-end circuitry ofports 108-1, . . . , 108-N. Thus, encoder circuit 118 receives first andsecond serial, digital data streams at inputs 120-1, . . . , 120-N.Encoder circuit 118 encodes the bits of the digital, serial data streamsinto a composite output signal that represents bits from both digital,serial data streams in the same symbol time as the serial data streams.For example, in one embodiment, encoder circuit 118 performs pulseamplitude modulation with four discrete levels for the serial input datastreams. In other embodiments, encoder circuit 118 encodes anyappropriate number of input data streams into a composite signal with,for example, any appropriate number of signal levels in the compositesignal. Optical modulator 124 receives the composite output signal andprovides a modulated optical signal for transmission over optical link106.

[0019] Optical receiver 104 receives the composite optical signal overoptical link 106 and reproduces the serial, digital signals at outputports 134-1, . . . , 134-N. Optical receiver 104 receives the compositeoptical signal at optical detector 126. In one embodiment, opticaldetector 126 comprises a photodiode or other appropriate circuit thatconverts optical signals to electric signals. Optical detector 126 iscoupled to decoder circuit 128 through gain circuit 130. In oneembodiment, gain circuit 130 includes an automatic gain control circuit.The automatic gain control circuit assures that the composite peak topeak voltage provided to decoder circuit 128 remains substantiallyconstant over the input dynamic range of optical receiver 104 and undermany other operating conditions of system 100. This allows decoder 128to properly reproduce the input signals for ports 108-1, . . . , 108-Nfrom the received composite signal. Alternatively, in other embodiments,decoder circuit 128 compensates for variations in the peak to peakvoltage provided by optical detector 126 when decoding the compositesignal.

[0020] Gain circuit 130 further includes a clock recovery mechanism thatprovides a clock signal to decoder circuit 128. This clock recoverymechanism is necessary to extract a clock signal to synchronize with theinputs at ports 108-1, . . . , 108-N for the purpose of outputting timealigned signals at output ports 134-1, . . . , 134-N.

[0021] Decoder circuit 128 receives the composite signal from opticaldetector 126 and gain circuit 130 and reproduces to the original signalsreceived at ports 120-1, . . . , 120-N. Thus, decoder circuit 128outputs a plurality of independent, serial, digital data streams atoutputs 134-1, . . . , 134-N. Each of the outputs 134-1, . . . , 134-Nis provided to a respective one of output ports 132-1, . . . , 132-Nthrough appropriate back end circuitry.

[0022] Due to the similarity in back end circuitry for ports 132-1, . .. 132-N, only the back end circuitry associated with port 132-1 isdescribed here. It is understood, however, that the back end circuitryassociated with the other ports is constructed and operates in a similarmanner. The back end circuitry associated with port 132-1 includesdeserializer 136, field programmable gate array 138, digital to analog(D/A) converter 140, and analog output circuit 142. Deserializer 136converts serial data streams from decoder circuit 128 into parallel datafor field programmable gate array 138. Field programmable gate array 138performs selected digital manipulation of the data, e.g., descrambling,decoding, and other appropriate digital processing operations. Theoutput of field programmable gate array 138 is provided to digital toanalog converter 140 in which analog output signals are generated basedon the digital input from field programmable gate array 138. Analogoutput circuit 142 further amplifies, filters, and otherwise processesthe analog signal for output port 132-1.

[0023]FIG. 2 is a schematic diagram of an embodiment of an encodercircuit, indicated generally at 200, for an optical transmitteraccording to the teachings of the present invention. Encoder circuit 200receives a plurality of digital, serial data streams and produces acomposite output signal based on the digital, serial data streams. Thecomposite output signal is used to drive an optical modulator fortransmitting the composite signal over an optical fiber. In thisembodiment, encoder circuit 200 receives two digital, serial datastreams at inputs 202 and 204 and produces a composite signal fortransmission over optical fiber 215. Encoder circuit 200 furtherincludes synchronous clock input 206 that provides a clock referencesignal for use in encoding the data received at inputs 202 and 204. Inthis embodiment, encoder circuit 200 encodes the two input data streamsinto a composite signal with four output levels based on the signallevel of the input signals according to the truth table provided below.Level of Reference Composite Number in Input 202 Input 204 Signal 0 0 1500 0 1 2 502 1 0 3 504 1 1 4 506

[0024] The signal levels of the composite signal are representedgraphically in FIG. 5. Encoder circuit 200 achieves the desired outputsignal level by controlling the current level provides to opticalmodulator 214.

[0025] Encoder circuit 200 includes three main components that generatethe output current used to drive optical modulator 214. These componentsinclude: encoder 208, latches 210, and modulator driver 212. Encoder 208comprises a logic circuit that is coupled to receive signals at inputs202 and 204. In this embodiment, encoder circuit 208 includes AND gate216, and exclusive-OR gates 218 and 220. AND gate 216 is coupled toinputs 202 and 204. Input 202 is also coupled to one input ofexclusive-OR gate 218. Input 204 is also coupled to one input ofexclusive-OR gate 220. An output of AND gate 216 is coupled to anotherinput of each of exclusive-OR gates 218 and 220. Encoder 208 providesthree output signals. These output signals are provided by AND gate 216,and exclusive-OR gates 218 and 220.

[0026] Latches 210 receive the output signals from encoder 208.Specifically, latches 210 comprise a plurality of latches 222, 224, and226. Each latch comprises a D flip-flop with a clock input coupled tosynchronous clock input 206.

[0027] Latches 210 are coupled to modulator driver 212. In thisembodiment, modulator driver 212 comprises a plurality of currentswitches 228, 230, and 232. Based on the output of latches 222, 224, and226, current switches 228, 230, and 232 provide a selected current levelto optical modulator 214 to produce output pulses with appropriateamplitudes based on the signals receive at input 202 and 204. In thisexample, current switch 228 is turned on to provide a current to opticalmodulator 214 when both input 202 and input 204 receive a logic 1.Further, current switch 230 is turned on to provide a different currentlevel to optical modulator 214 when input 202 is a logic 1 and input 204is a logic 0. Finally, current switch 232 is turned on to provide theircurrent level to optical modulator 214 when input 202 is a logic 0 andinput 204 is a logic 1. When input 202 and input 204 both receive alogic 0, only bias current is provided to optical modulator 214 thusproducing a fourth output level.

[0028] In one embodiment, optical modulator 214 includes laser 238 andelectro-absorptive modulator 236. Optical modulator 214 is furthercoupled to laser cooler and bias control circuit 240.

[0029] The operation of encoder circuit 200 is described with respect totiming diagrams shown in FIGS. 3A-3G. In the timing diagrams, thesignals are represented as “return to zero” signals. It is understood,however, that in other embodiment, the input signals to the encodercircuit do not return to zero on each bit.

[0030]FIG. 3A illustrates a stream of pulses for a synchronous referenceclock for use in this example. FIGS. 3B and 3C provide an example ofinput signals provided to inputs 202 and 204, respectively, of encodercircuit 200. FIGS. 3D, 3E, and 3F illustrate the states of currentswitches 228, 230, and 232, respectively. Finally, FIG. 3G illustratesthe power level of the optical output of optical modulator 214. Todescribe the operation of encoder circuit 200, an example of each of thepossible power output levels in FIG. 3G are provided in turn below.

[0031] Optical power level 4 is achieved when both input 202 and input204 receive a logic 1 as in pulse 1 of this example. In this case, ANDgate 216 produces a logic 1 output. This logic 1 output is latched bylatch 222 and turns on current switch 228 as indicated in FIG. 3D. It isnoted that current switches 230 and 232 are turned off as indicated inFIGS. 3E and 3F. Thus, the current from current switch 228 drives thevoltage on the resister 234 and sets the output pulse level of modulator214 as indicated in pulse 1 of FIG. 3G. In one embodiment, this opticalpower level corresponds to 0 dB relative attenuation.

[0032] Optical power level 3 is achieved when input 202 receives a logiclevel 1 and input 204 receives a logic 0 as in pulse 4 of this example.In this case, exclusive-OR gate 218 is the only logic gate that producesa logic 1 output. This logic 1 output is latched by latch 224 and turnson current switch 230 as indicated in FIG. 3E. In this case, switches228 and 232 are both turned off as indicated in FIGS. 3D and 3F. Thecurrent from current switch 230 drives the voltage on the resister 234and sets the output pulse level of modulator 214 as indicated in pulse 4of FIG. 3G. In one embodiment, this optical power level corresponds to−1.75 dB relative attenuation.

[0033] Optical power level 2 is achieved when input 202 receives a logiclevel 0 and input 204 receives a logic 1 as in pulse 3 of this example.In this case, exclusive-OR gate 220 is the only logic gate that producesa logic 1 output. This logic 1 output is latched by latch 226 and turnson current switch 232 as indicated in FIG. 3F. In this case, switches228 and 230 are both turned off as indicated in FIGS. 3D and 3E. Thecurrent from current switch 232 drives the voltage on the resister 234and sets the output pulse level of modulator 214 as indicated in pulse 3of FIG. 3G. In one embodiment, this optical power level corresponds to−4.75 dB relative attenuation.

[0034] Optical power level 1 is achieved when input 202 receives a logiclevel 0 and input 204 receives a logic 0 as in pulse 5 of this example.In this case, none of the logic gates produce a logic 1 output. Thus,all of the current switches 228, 230, and 232 are turned off asindicated in FIGS. 3D, 3E, and 3F. The fixed, low level bias currentdrives the voltage on the resister 234 and sets the output pulse levelof modulator 214 as indicated in pulse 5 of FIG. 3G. In one embodiment,this optical power level corresponds to −15 dB relative attenuation.

[0035]FIG. 4 is a schematic diagram of an embodiment of a decodercircuit, indicated generally at 400, for an optical receiver accordingto the teachings of the present invention. Decoder circuit 400 receivesa composite optical signal and reproduces a plurality of digital, serialdata streams from the composite signal. The composite input signal isreceived over optical fiber 402. In this embodiment, decoder circuit 400produces two digital, serial data streams at outputs 404 and 406.Decoder circuit 400 decodes a composite signal with four output levelsinto the two independent serial, data streams at outputs 404 and 406. Itis understood that in other embodiments other appropriateencoding/decoding schemes are used to allow any appropriate number ofindependent, serial, digital data streams to be reproduced from acomposite signal.

[0036] Decoder circuit 400 receives the composite signal from fiberoptic cable 402 through optical receiver 408 and amplifier 414. Opticalreceiver 408, in one embodiment, includes photodiode 410 andpreamplifier 412. Preamplifier 412 has a gain that is selected to keepthe operation linear over the full input dynamic range. Preamplifier 412is coupled to amplifier 414. Amplifier 414 is coupled to input 415 ofdecoder circuit 400. In one embodiment, automatic gain control circuit416 is coupled to selectively adjust the gain of amplifier 414. Inpractice, automatic gain control circuit 416 attempts to maintain thepeak-to-peak voltage of the composite signal into input 415 atsubstantially a constant level. Input 415 is also provided to clockrecovery circuit 418. Clock recovery circuit 418 derives a clock signalfrom the composite signal. This clock signal is provided as anotherinput to decoder circuit 400 at 417. Decoder circuit 400, in oneembodiment, includes three stages. Each of these stages is discussed inturn below.

[0037] Decoder circuit 400 includes a plurality of comparators 420. Inthis example, decoder circuit 400 includes three comparators 426, 428,and 430, respectively, in order to detect the four possible levels ofthe composite input signal. In other embodiments, an appropriate numberof comparators is used to allow decoder circuit 400 to discern properlybetween the various levels of the composite input signal to allow theoriginal signals to be reproduced at outputs 404 and 406. Each of thecomparators 426, 428, and 430 are implemented as D flip-flops with the{overscore (D)} input coupled to a reference voltage. For example,comparator 430 is coupled to the reference voltage V3, comparator 428 iscoupled to the reference voltage V2 and the comparator 426 is coupled toreference voltage V1. The reference voltages are established by aplurality of resistors R1, R2, R3, and R4 that are coupled between Vccand ground. FIG. 5 demonstrates the relationship between the referencevoltages, V1, V2, and V3, and the voltage levels used for encoding data,namely, levels 500, 502, 504, and 506.

[0038] Decoder circuit 400 also includes decoder 422. Decoder 422 is alogic circuit that, in this embodiment, includes three logic gates.Decoder 422 includes AND gate 432 and exclusive-OR gates 434 and 436.AND gate 432 is coupled to the Q output of each of comparators 428 and430. Exclusive-OR gate 434 is also coupled to the Q output of each ofcomparators 428 and 430. Exclusive-OR gate 436 is coupled to the Qoutput of comparator 426 and to the output of exclusive-OR gate 434. Theoutput of exclusive-OR gate 436 and the output of AND gate 432 providethe outputs for decoder 422.

[0039] Decoder circuit 400 further includes latches 424 which providethe output for decoder circuit 400. Latches 424 include first and secondD flip-flops 438 and 440. The clock input of each of D flip-flops 438and 440 are coupled to the recovered clock signal at input 417. First Dflip-flop 438 is coupled to the output of exclusive-OR gate 436.Similarly, second D flip-flop 440 is coupled to the output of AND gate432. Flip-flops 438 and 440 are coupled to outputs 404 and 406,respectively, of decoder circuit 400.

[0040] The operation of decoder circuit 400 is described with respect toan example provided in FIGS. 6A-6G. In the timing diagrams, the signalsare represented as “return to zero” signals. It is understood, however,that in other embodiment, the bits of the serial, digital data streamsdo not return to zero on each bit.

[0041]FIG. 6A illustrates a stream of pulses for a recovered referenceclock for use in this example. FIG. 6B provides an example of acomposite signal output from an optical detector. FIGS. 6C, 6D and 6Eillustrate the outputs of comparators 426, 428, and 430, respectively.Finally, FIGS. 6F and 6G illustrate the serial, digital data streamsreproduced from the composite signal at outputs 404 and 406,respectively. To describe the operation of decoder circuit 500, anexample of each of the possible power input levels in FIG. 6B areprovided in turn below.

[0042] An optical power level 4 corresponding to a logic 1 at bothoutputs 404 and 406 is shown for example with respect to pulse 1 in thecomposite signal of FIG. 6B. When a pulse of level 4 is received, all ofthe comparators 426, 428, and 430 provide high logic level outputsignals. With two logic 1 inputs, AND gate 432 produces a logic 1 outputand thus output 406 is a logic 1. Further, with both of the inputs toexclusive-OR gate 434 at logic 1, exclusive-OR gate 434 outputs a logic0. Exclusive-OR gate 436 receives a logic 0 from exclusive-OR gate 434and a logic 1 from comparator 426. Thus, output 404 is also a logic 1.

[0043] An optical power level 3 corresponding to a logic 0 at output 404and a logic 1 at output 406 is shown for example with respect to pulse 4in the composite signal of FIG. 6B. When a pulse of level 3 is received,comparators 428 and 430 provide a high logic level output signal andcomparator 426 provides a logic 0 as shown in FIGS. 6D, 6E, and 6C,respectively. With two logic 1 inputs, AND gate 432 produces a logic 1output and thus output 406 is a logic 1. Further, with both of theinputs to exclusive-OR gate 434 at logic 1, exclusive-OR gate 434outputs a logic 0. Exclusive-OR gate 436 receives a logic 0 fromexclusive-OR gate 434 and a logic 0 from comparator 426. Thus, output404 is also a logic 0.

[0044] An optical power level 2 corresponding to a logic 1 at output 404and a logic 0 at output 406 is shown for example with respect to pulse 3in the composite signal of FIG. 6B. When a pulse of level 2 is received,comparator 430 provides a logic 1 output signal and comparators 426 and428 provide a logic 0 as shown in FIGS. 6E, 6C, and 6D, respectively.With one logic 1 input, AND gate 432 produces a logic 0 output and thusoutput 406 is a logic 0. Further, with one of the inputs to exclusive-ORgate 434 at logic 1, exclusive-OR gate 434 outputs a logic 1. Thus,exclusive-OR gate 436 receives a logic 1 from exclusive-OR gate 434 anda logic 0 from comparator 426. Thus, output 404 is also a logic 1.

[0045] An optical power level 1 corresponding to a logic 0 at output 404and a logic 0 at output 406 is shown for example with respect to pulse 5in the composite signal of FIG. 6B. When a pulse of level 1 is received,comparators 426, 428 and 430 provide logic 0 output signals as shown inFIGS. 6C, 6D, and 6E, respectively. With two logic 0 inputs, AND gate432 produces a logic 0 output and thus output 406 is a logic 0. Further,with both of the inputs to exclusive-OR gate 434 at logic 0,exclusive-OR gate 434 outputs a logic 0. Thus, exclusive-OR gate 436receives a logic 0 from exclusive-OR gate 434 and a logic 0 fromcomparator 426. Thus, output 404 is also a logic 0.

[0046] Although specific embodiments have been illustrated and describedin this specification, it will be appreciated by those of ordinary skillin the art that any arrangement that is calculated to achieve the samepurpose may be substituted for the specific embodiment shown. Thisapplication is intended to cover any adaptations or variations of thepresent invention. For example, other techniques for encoding multiplestreams of data into a composite signal can be used in place of pulseamplitude modulation, e.g., coding techniques which use M signal levelsfor N inputs wherein M can equal N. Further, other voltage levels,signal levels and numbers of bit streams can be used.

What is claimed is:
 1. An optical communication system, comprising: anoptical fiber; an optical transmitter, including: a first input adaptedto receive serial, digital data from a first source, at least oneadditional input adapted to receive serial, digital data from at leastone additional source, an encoder circuit, coupled to the first and theat least one additional inputs, that produces an output signal with aselected level based on the serial, digital data from the first and theat least one additional sources, and an optical modulator, coupled tothe encoder circuit, that produces a pulse amplitude modulated opticalsignal based on the levels of the signal from the encoder circuit andthat provides the pulse amplitude modulated optical signal to theoptical fiber for transmission; and an optical receiver, including: anoptical detector, coupled to the optical fiber, that receives the pulseamplitude modulated optical signal and produces an electrical signalwith selected levels based on the pulse amplitude modulated signal; adecoder circuit, coupled to the optical detector, that reproduces theserial, digital data from the first and the at least one additional datasources based on the electrical signal from the optical detector, afirst output and at least one additional output, coupled to the decodercircuit, the first and the at least one additional output provide thereproduced digital, serial data for the first and the at least oneadditional source, respectively.
 2. The system of claim 1, wherein theencoder circuit comprises: an encoder, responsive to the serial, digitaldata from the first and the at least one additional sources, thatproduces a plurality of output signals based on the serial, digital datafrom the first and the at least one additional sources; a plurality oflatches, responsive to the plurality of output signals from the encoder,that latch the output signals on a clock signal; and a plurality ofcurrent switches, responsive to the plurality of latches, that provideselected current levels to the optical modulator to control the level ofthe pulse amplitude modulation based on the serial, digital data fromthe first and the at least one additional sources.
 3. The system ofclaim 1, wherein the encoder circuit comprises a circuit that producesan output signal with a selected current level based on the serial,digital data from the first and the at least one additional sources. 4.The system of claim 1, wherein the encoder circuit comprises a circuitthat produces an output signal with one of N current levels based onserial, digital data from M sources.
 5. The system of claim 1, whereinthe encoder circuit comprises a circuit that produces an output signalwith one of 2^(M) current levels based on serial, digital data from Msources.
 6. The system of claim 1, wherein the encoder circuit comprisesa circuit that produces an output signal with one of M current levelsbased on serial, digital data from M sources.
 7. The system of claim 1,wherein the optical modulator comprises a laser with anelectro-absorptive modulator.
 8. The system of claim 1, wherein theoptical modulator comprises a directly modulated laser.
 9. The system ofclaim 1, wherein the optical receiver further includes an automatic gaincontrol circuit coupled between the optical detector and the decodercircuit that maintains a substantially constant peak to peak level tothe decoder circuit.
 10. The system of claim 1, wherein the decodercircuit comprises: a plurality of comparators, responsive to theelectrical signal from the optical detector, each comparator adapted tocompare the electrical signal with at least one selected level; adecoder, coupled to the comparators, that reproduces the serial, digitaldata from the first and at least one additional sources based on thecomparisons; and a plurality of latches, coupled to the decoder, thatprovide the reproduced serial, digital data for the first and the atleast one additional sources.
 11. The system of claim 10, wherein theplurality of comparators each selectively compare the electrical signalfrom the optical detector with an adaptive reference level, wherein thereference level is adapted based on peak to peak variations in theelectrical signal.
 12. The system of claim 1, wherein the decodercircuit comprises a decoder circuit that compares the electrical signalfrom the optical detector with a plurality of signal levels.
 13. Thesystem of claim 1, wherein the decoder circuit comprises a decodercircuit that compares the electrical signal with N levels for M sources.14. The system of claim 1, wherein the decoder circuit comprises adecoder circuit that compares the electrical signal with 2^(M)−1 levelsfor M sources.
 15. The system of claim 1, wherein the decoder circuitcomprises a decoder circuit that compares the electrical signal with Mlevels for M sources.
 16. An optical communication system, comprising:an optical fiber; an optical transmitter, coupled to the optical fiber,the optical transmitter adapted to encode a composite optical signalbased at least two, independent input signals; and an optical receiver,coupled to the optical fiber, the optical receiver adapted to decode thecomposite optical signal to reproduce the at least two, independentinput signals as output signals.
 17. The system of claim 16, wherein theoptical transmitter includes an encoder circuit, the encoder circuitcomprises: an encoder, responsive to the at least two, independent inputsignals, that produces a plurality of output signals based on the atleast two, independent input signals; a plurality of latches, responsiveto the plurality of output signals from the encoder, that latch theoutput signals on a clock signal; and a plurality of current switches,responsive to the plurality of latches, that provide selected currentlevels to an optical modulator to control the level of a pulse amplitudemodulated optical signal based on the at least two, independent inputsignals.
 18. The system of claim 16, wherein the optical transmitterincludes an encoder circuit that produces an output signal with aselected current level to drive an optical modulator based on the atleast two, independent input signals.
 19. The system of claim 18,wherein the encoder circuit comprises a circuit that produces an outputsignal with one of N current levels based on M independent inputsignals.
 20. The system of claim 18, wherein the encoder circuitcomprises a circuit that produces an output signal with one of 2^(M)current levels based M independent input signals.
 21. The system ofclaim 18, wherein the encoder circuit comprises a circuit that producesan output signal with one of M current levels based M independent inputsignals.
 22. The system of claim 16, wherein the optical transmitterincludes a laser with an electro-absorptive modulator.
 23. The system ofclaim 16, wherein the optical transmitter includes a directly modulatedlaser.
 24. The system of claim 16, wherein the optical receiver furtherincludes an automatic gain control circuit coupled between an opticaldetector and a decoder circuit that maintains a substantially constantpeak to peak level to the decoder circuit.
 25. The system of claim 16,wherein the optical receiver comprises a decoder circuit with aplurality of comparators that are each adapted to compare a receivedelectrical signal with at least one selected level to reproduce the atleast two, independent input signals.
 26. The system of claim 24,wherein the plurality of comparators each selectively compare theelectrical signal with an adaptive reference level, wherein thereference level is adapted based on peak to peak variations in theelectrical signal.
 27. The system of claim 16, wherein the opticalreceiver includes a decoder circuit that compares an electrical signalwith N levels for M sources.
 28. The system of claim 16, wherein theoptical receiver includes a decoder circuit that compares an electricalsignal with 2^(M)−1 levels for M sources.
 29. The system of claim 16,wherein the optical receiver includes a decoder circuit that compares anelectrical signal with M levels for M sources.
 30. An opticaltransmitter, including: a first input adapted to receive serial, digitaldata from a first source, at least one additional input adapted toreceive serial, digital data from at least one additional source, anencoder circuit, coupled to the first and the at least one additionalinputs, that produces an output signal with a selected level based onthe serial, digital data from the first and the at least one additionalsources, and an optical modulator, coupled to the encoder circuit, thatproduces a pulse amplitude modulated optical signal based on the levelsof the signal from the encoder circuit and that provides the pulseamplitude modulated optical signal to an optical output fortransmission.
 31. The optical transmitter of claim 30, wherein theencoder circuit comprises: an encoder, responsive to the serial, digitaldata from the first and the at least one additional sources, thatproduces a plurality of output signals based on the serial, digital datafrom the first and the at least one additional sources; a plurality oflatches, responsive to the plurality of output signals from the encoder,that latches the output signals on a clock signal; and a plurality ofcurrent switches, responsive to the plurality of latches, that provideselected current levels to the optical modulator to control the level ofthe pulse amplitude modulation based on the serial, digital data fromthe first and the at least one additional sources.
 32. The opticaltransmitter of claim 30, wherein the encoder circuit comprises a circuitthat produces an output signal with a selected current level based onthe serial, digital data from the first and the at least one additionalsources.
 33. The optical transmitter of claim 30, wherein the encodercircuit comprises a circuit that produces an output signal with one of Ncurrent levels based on serial, digital data from M sources.
 34. Theoptical transmitter of claim 30, wherein the encoder circuit comprises acircuit that produces an output signal with one of 2^(M) current levelsbased on serial, digital data from M sources.
 35. The opticaltransmitter of claim 30, wherein the encoder circuit comprises a circuitthat produces an output signal with one of M current levels based onserial, digital data from M sources.
 36. The optical transmitter ofclaim 30, wherein the optical modulator comprises a laser with anelectro-absorptive modulator.
 37. The optical transmitter of claim 30,wherein the optical modulator comprises a directly modulated laser. 38.An optical transmitter, including: a first input adapted to receiveserial, digital data from a first source, at least one additional inputadapted to receive serial, digital data from at least one additionalsource, an encoder circuit, coupled to the first and the at least oneadditional inputs, that encodes the serial, digital data from the firstand the at least one additional sources into an electrical signal, andan optical modulator, coupled to the encoder circuit, that produces apulse amplitude modulated optical signal based on the electrical signalfrom the encoder circuit and that provides the pulse amplitude modulatedoptical signal to an optical output for transmission.
 39. The opticaltransmitter of claim 38, wherein the encoder circuit provides a selectedcurrent level to the optical modulator to control the level of the pulseamplitude modulation based on the serial, digital data from the firstand the at least one additional sources.
 40. The optical transmitter ofclaim 38, wherein the encoder circuit comprises a circuit that producesan output signal with one of N current levels based on serial, digitaldata from M sources.
 41. The optical transmitter of claim 38, whereinthe encoder circuit comprises a circuit that produces an output signalwith one of 2^(M) current levels based on serial, digital data from Msources.
 42. The optical transmitter of claim 38, wherein the encodercircuit comprises a circuit that produces an output signal with one of Mcurrent levels based on serial, digital data from M sources.
 43. Theoptical transmitter of claim 38, wherein the optical modulator comprisesa laser with an electro-absorptive modulator.
 44. The opticaltransmitter of claim 38, wherein the optical modulator comprises adirectly modulated laser.
 45. An optical receiver, comprising: anoptical input, adapted to be coupled to an optical fiber; an opticaldetector, coupled to the optical input, that receives a pulse amplitudemodulated optical signal and produces an electrical signal with selectedlevels based on the pulse amplitude modulated signal; a decoder circuit,coupled to the optical detector, that produces a first and at least oneadditional serial, digital data stream based on the electrical signalfrom the optical detector, a first output and at least one additionaloutput, coupled to the decoder circuit, the first and the at least oneadditional output provide the first and the at least one additionaldigital, serial data stream, respectively.
 46. The optical receiver ofclaim 45, and further including an automatic gain control circuitcoupled between the optical detector and the decoder circuit thatmaintains a substantially constant peak to peak level to the decodercircuit.
 47. The optical receiver of claim 45, wherein the decodercircuit comprises: a plurality of comparators, responsive to theelectrical signal from the optical detector, each comparator adapted tocompare the electrical signal with at least one selected level; adecoder, coupled to the comparators, that produces the first and the atleast one additional serial, digital data streams based on thecomparisons; and a plurality of latches, coupled to the decoder, thatprovide the first and the at least one additional serial, digital datastreams to the first and the at least one additional output.
 48. Theoptical receiver of claim 47, wherein the plurality of comparators eachselectively compare the electrical signal from the optical detector withan adaptive reference level, wherein the reference level is adaptedbased on peak to peak variations in the electrical signal.
 49. Theoptical receiver of claim 45, wherein the decoder circuit comprises adecoder circuit that compares the electrical signal from the opticaldetector with a plurality of signal levels.
 50. The optical receiver ofclaim 45, wherein the decoder circuit comprises a decoder circuit thatcompares the electrical signal with N levels for M serial, digital datastreams.
 51. The optical receiver of claim 45, wherein the decodercircuit comprises a decoder circuit that compares the electrical signalwith 2^(M)−1 levels for M serial, digital data streams.
 52. The opticalreceiver of claim 45, wherein the decoder circuit comprises a decodercircuit that compares the electrical signal with M levels for M serial,digital data streams.
 53. An optical receiver, comprising: an opticalinput, adapted to be coupled to an optical fiber; an optical detector,coupled to the optical input, that receives a pulse amplitude modulatedoptical signal and produces an electrical signal based on the pulseamplitude modulated signal; a decoder circuit, coupled to the opticaldetector, that produces a first and at least one additional serial,digital data stream based on the electrical signal from the opticaldetector.
 54. The optical receiver of claim 53, and further including anautomatic gain control circuit coupled between the optical detector andthe decoder circuit that maintains a substantially constant peak to peaklevel to the decoder circuit.
 55. The optical receiver of claim 53,wherein the decoder circuit comprises: a plurality of comparators,responsive to the electrical signal from the optical detector, eachcomparator adapted to compare the electrical signal with at least oneselected level; a decoder, coupled to the comparators, that produces thefirst and the at least one additional serial, digital data streams basedon the comparisons; and a plurality of latches, coupled to the decoder,that provide the first and the at least one additional serial, digitaldata streams to the first and the at least one additional output. 56.The optical receiver of claim 55, wherein the plurality of comparatorseach selectively compare the electrical signal from the optical detectorwith an adaptive reference level, wherein the reference level is adaptedbased on peak to peak variations in the electrical signal.
 57. Theoptical receiver of claim 53, wherein the decoder circuit comprises adecoder circuit that compares the electrical signal from the opticaldetector with a plurality of signal levels.
 58. The optical receiver ofclaim 53, wherein the decoder circuit comprises a decoder circuit thatcompares the electrical signal with N levels for M serial, digital datastreams.
 59. The optical receiver of claim 53, wherein the decodercircuit comprises a decoder circuit that compares the electrical signalwith 2^(M)−1 levels for M serial, digital data streams.
 60. The opticalreceiver of claim 53, wherein the decoder circuit comprises a decodercircuit that compares the electrical signal with M levels for M serial,digital data streams.
 61. A method for encoding serial, digital data,the method comprising: receiving a first serial, digital data stream;receiving at least one additional serial, digital data stream; producingan encoded signal with levels based on the first and the at least oneadditional serial, digital data streams; producing a pulse amplitudemodulated optical signal based on the levels of the encoded signal;transmitting the pulse amplitude modulated optical signal.
 62. Themethod of claim 61, wherein producing an encoded signal comprisesproducing a signal with current levels based on the first and the atleast one additional serial, digital data streams.
 63. The method ofclaim 61, wherein producing an encoded signal comprises producing asignal with current levels selected from N current levels for M serial,digital data streams.
 64. The method of claim 61, wherein producing anencoded signal comprises producing a signal with current levels selectedfrom 2^(M) current levels for M serial, digital data streams.
 65. Themethod of claim 61, wherein producing an encoded signal comprisesproducing a signal with current levels selected from M current levelsfor M serial, digital data streams.
 66. The method of claim 61, whereinproducing a pulse amplitude modulated optical signal comprises producinga pulse amplitude modulated optical signal with a directly modulatedlaser.
 67. A method for transmitting a plurality of serial, digital datastreams over an optical fiber link, the method comprising: receiving theplurality of serial, digital data streams; producing a pulse amplitudemodulated optical signal based on the plurality of serial, digital datastreams; and transmitting the pulse amplitude modulated optical signal.68. The method of claim 67, wherein producing the pulse amplitudemodulated optical signal comprises: encoding the plurality of serial,digital data streams as an electrical signal with a plurality of signallevels; and modulating an optical signal with the electrical signal. 69.The method of claim 67, wherein producing a pulse amplitude modulatedoptical signal comprises generating an optical signal with signal levelsselected from N signal levels for M serial, digital data streams. 70.The method of claim 67, wherein producing a pulse amplitude modulatedoptical signal comprises generating an optical signal with signal levelsselected from 2^(M) signal levels for M serial, digital data streams.71. The method of claim 67, wherein producing a pulse amplitudemodulated optical signal comprises generating an optical signal withsignal levels selected from M signal levels for M serial, digital datastreams.
 72. A method for decoding a plurality of serial, digital datastreams from an optical signal, the method comprising: receiving theoptical signal, wherein the optical signal is a pulse amplitudemodulated signal; converting the optical signal to an electrical signal;comparing the electrical signal with a plurality of levels; andgenerating the plurality of serial, digital data streams based on thecomparison with the plurality of levels.
 73. The method of claim 72, andfurther comprising selectively adjusting the peak to peak level of theelectrical signal prior to comparing.
 74. The method of claim 72,wherein comparing the electrical signal with a plurality of levelscomprises comparing the electrical signal with N levels for M serial,digital data streams.
 75. The method of claim 72, wherein comparing theelectrical signal with a plurality of levels comprises comparing theelectrical signal with 2^(M)−1 levels for M serial, digital datastreams.
 76. The method of claim 72, wherein comparing the electricalsignal with a plurality of levels comprises comparing the electricalsignal with M levels for M serial, digital data streams.
 77. An opticaltransmitter, including: a first input adapted to receive serial, digitaldata from a first source, at least one additional input adapted toreceive serial, digital data from at least one additional source, anencoder circuit, coupled to the first and the at least one additionalinputs, that produces an output signal with a selected level based onthe serial, digital data from the first and the at least one additionalsources, and an optical modulator, coupled to the encoder circuit, thatproduces a composite optical signal based on the levels of the signalfrom the encoder circuit and that provides the composite optical signalto an optical output for transmission.